Bhagavad Gita 1-12

tasya saMjayan harSham kuruvRuddhaHa pitamahaHa

siMhanaadaM vinadyOchchaiHi shaMkhaM dadmou pratapavaan ||12||

12-1 "In order to cheer him up the eldest of the Kauravas, the great grandfather makes a lions roar and blew his conch shell with vigor."

12-2 "In the mean time the eldest of the kauravas, the great grandfather makes a lions roar and blew his conch shell with such vigor it made Duryodhana very happy."

The translation can be made either way. And both of them convey a different meaning.

12-1 - Bhishma was listening to the conversation between Duryodhana and Dronacharya. He understood the anxiety that Duryodhana was experiencing. Bhishma conveys to Duryodhana that he is not weak and he still has such a capacity that he can terrify the enemy camp just by his lions roar and the sound of his conch shell. He also meant that once in the battlefield a real Kshatriya would not worry with anxiety but only fights with valor. This he did either to reduce the anxiety of Duryodhana or to ridicule his anxiety.

12-2 - Bhishma was not listening to the conversation between Duryodhana and Dronacharya. He being the CIC and eldest of all the Kauravas had to initiate the battle by his lions roar and blowing his conch shell. That was the protocol. Bhishma was just following the protocol as he was a person who abided by the protocols, rules and regulations throughout his life. Nothing mattered to him as long as he could keep up to the word given by him - even if he had to fight on the side of adharma. He was bound by his own words/Pratigna.